Acidizing a hydrocarbon formation is the process of reacting an acid in the formation to enhance the flow of hydrocarbons to the wellbore. This can be through the dissolution of existing formation material or through the removal or bypass of blockage, often referred to as “damage to the well” which can be caused by natural or man-made conditions. Acidizing, or acid stimulation, opens up the channels around the wellbore, thereby improving the flow rate. Early attempts at acidizing formations focused on the injection of a simple acidic solution into a wellbore. Such attempts proved to be inefficient as the fluid would often react or spend too quickly; and therefore be incapable of penetrating deep into the formation, thereby limiting effectiveness to very near-wellbore applications. Also, many subterranean formations contain sections with varying permeability. When acid is injected into a formation, it typically acidizes the section of the formation with the highest permeability and which has the highest degree of water saturation. The acidizing formulations of the prior art lacked a control mechanism to divert or force the acid from the higher permeability and/or water saturated sections of the formation to the lower permeability or oil bearing sections.
Though several alternatives have emerged, they have each failed to precisely control the flow of the acidizing fluid. One such alternative, disclosed in U.S. Patent Application No. 2002/0147114, is directed to the use of a single surfactant system as a gelled acidizing fluid wherein the surfactant gels an acid fluid containing 3-15% HCl solution by volume. Extra energy is often required to pump this already viscous gelled fluid into the well.
Another alternative disclosed in the prior art used N,N,-bis(2-hydroxyethyl) tallow ammonium acetate. Unfortunately, this gelling agent exhibited breakdown at higher temperatures as the acid was spent. Also, this gelling agent gelled too quickly. Therefore, there could not be maximum penetration into the formation. Additionally, the maximum viscosity was too low to adequately perform the necessary diverting.
Additionally, some of the alternatives of the prior art employed crosslinked systems. As such, as the fluid was pumped into the formation, a polymerization reaction occurred which gelled the fluid. These systems left a residue in the formation, which further damaged the formation. Such systems were further dependent upon a sensitive chemical reaction. The polymerization was very difficult to optimize so that it was delayed during pumping but maximized once in the formation. Once these systems were in place, they were difficult to remove. To do so required that they be un-crosslinked.
Other attempts at creating a gelled acidizing fluid have used a multi-surfactant based system. An example of this type of system was described in U.S. Pat. No. 6,399,546. These systems were not desirable for many reasons. First, these systems required mixing of two or more compounds at the well site, which led to quality control issues. The ratio of the ingredients often depended on the temperature and the pH of the system, which further led to quality control issues. Further, the gelling process as described in U.S. Pat. No. 6,399,546 required the introduction of a chemical trigger in order to cause the gelling process.
Therefore, there exists a need to develop an acidizing system that does not rely upon crosslinking for gelation. The viscosity of such acidizing systems should further be capable of being controlled internally. The control of the system should be simple and not based on multiple factors, particularly temperature and chemical triggers. Desirably, the system should be capable of being delayed to allow maximum penetration and further exhibit a high viscosity to adequately divert incoming fluids. The system should further be stable at high temperature while the acid is being spent.